Individual and combined additions of calcium and antimony on microstructure and mechanical properties of squeeze-cast AZ91D magnesium alloy

The effects of combined additions of Ca and Sb on the microstructure and tensile properties of AZ91D alloy fabricated by squeeze-casting have been investigated. For comparison, the same has also been studied with and without individual additions of Ca and Sb. The results indicate that both individual and combined additions refine the grain size and beta-Mg17Al12 phase, which is more pronounced with combined additions. Besides alpha-Mg and beta-Mg17Al12 phases, a new reticular Al2Ca and rod-shaped Mg3Sb2 phases are formed following individual additions of Ca and Sb in the AZ91D alloy. With combined additions, an additional Ca2Sb phase is formed suppressing Mg3Sb2 phase. Additions of both Ca and Sb increase yield strength (YS) at both ambient and elevated temperatures up to 200 degrees C. However, both ductility and ultimate tensile strength (UTS) decrease first up to 150 degrees C and then increase at 200 degrees C. The increase in YS is attributed to the refinement of grain size, whereas, ductility and UTS are deteriorated by the presence of brittle Al2Ca, Mg3Sb2 and Ca2Sb phases. The best tensile properties are obtained in the AZXY9110 alloy owing to the presence of lesser amount of brittle Al2Ca and Ca2Sb phases resulted from the optimum content of 1.0Ca and 0.3Sb (wt%). The fracture surface of the tensile specimen tested at ambient temperature reveals cleavage failure that changes to quasi-cleavage at 200 degrees C. The squeeze-cast alloys exhibited better tensile properties as compared to that of the gravity-cast alloys nullifying the detrimental effects of Ca and/or Sb additions. (C) 2014 Elsevier B.V. All rights reserved.

Effects of temperature and clay content on water absorption characteristics of modified MMT clay/cyclic olefin copolymer nanocomposite films: Permeability, dynamic mechanical properties and the encapsulated organic device performance

In the present study, amino-silane modified layered organosilicates were used to reinforce cyclic olefin copolymer to enhance the thermal, mechanical and moisture impermeable barrier properties. The optimum clay loading (4%) in the nanocomposite increases the thermal stability of the film while further loading decreases film stability. Water absorption behavior at 62 degrees C was carried out and compared with the behavior at room temperature and 48 degrees C. The stiffness of the matrix increases with clay content and the recorded strain to failure for the composite films was lower than the neat film. Dynamic mechanical analysis show higher storage modulus and low loss modulus for 2.5-4 wt% clay loading. Calcium degradation test and device encapsulation also show the evidence of optimum clay loading of 4 wt% for improved low water vapor transmission rates compared to other nanocomposite films. (C) 2014 Elsevier Ltd. All rights reserved.

Experimental studies on the influence of intermediate principal stress and inclination on the mechanical behaviour of angular sands

A comprehensive experimental study has been made on angular sand to investigate various aspects of mechanical behavior. A hollow cylinder torsion testing apparatus is used in this program to apply a range of stress conditions on this angular quartzitic fine sand under monotonic drained shear. The effect of the magnitude and inclination of the principal stresses on an element of sand is studied through these experiments. This magnitude and inclination of the principal stresses are presented as an ``ensemble measure of fabric in sands''. This ensemble measure of fabric in the sands evolves through the shearing process, and reaches the final state, which indeed has a unique fabric. The sand shows significant variation in strength with changing inclination of the principal stresses. The locus of the final stress state in principal stress space is also mapped from these series of experiments. Additional aspects of non-coaxiality, a benchmarking exercise with a few constitutive models is presented here. This experimental approach albeit indirect shows that a unique state which is dependent on the fabric, density and confining stress exists. This suite of experiments provides a well-controlled data set for a clear understanding on the mechanical behavior of sands.

Conservation Properties of the Trapezoidal Rule in Linear Time Domain Analysis of Acoustics and Structures

The trapezoidal rule, which is a special case of the Newmark family of algorithms, is one of the most widely used methods for transient hyperbolic problems. In this work, we show that this rule conserves linear and angular momenta and energy in the case of undamped linear elastodynamics problems, and an ``energy-like measure'' in the case of undamped acoustic problems. These conservation properties, thus, provide a rational basis for using this algorithm. In linear elastodynamics problems, variants of the trapezoidal rule that incorporate ``high-frequency'' dissipation are often used, since the higher frequencies, which are not approximated properly by the standard displacement-based approach, often result in unphysical behavior. Instead of modifying the trapezoidal algorithm, we propose using a hybrid finite element framework for constructing the stiffness matrix. Hybrid finite elements, which are based on a two-field variational formulation involving displacement and stresses, are known to approximate the eigenvalues much more accurately than the standard displacement-based approach, thereby either bypassing or reducing the need for high-frequency dissipation. We show this by means of several examples, where we compare the numerical solutions obtained using the displacement-based and hybrid approaches against analytical solutions.

Tunable mechanical and thermal properties of ZnS/CdS core/shell nanowires

Using all-atom molecular dynamics (MD) simulations, we have studied the mechanical properties of ZnS/CdS core/shell nanowires. Our results show that the coating of a few-atomic-layer CdS shell on the ZnS nanowire leads to a significant change in the stiffness of the core/shell nanowires compared to the stiffness of pure ZnS nanowires. The binding energy between the core and shell region decreases due to the lattice mismatch at the core-shell interface. This reduction in binding energy plays an important role in determining the stiffness of a core/shell nanowire. We have also investigated the effects of the shell on the thermal conductivity and melting behavior of the nanowires.

Deformation behaviour of Rheocast A356 Al alloy at microlevel considering approximated RVEs

A micromechanical approach is considered here to predict the deformation behaviour of Rheocast A356 (Al-Si-Mg) alloy. Two representative volume elements (RVEs) are modelled in the finite element (FE) framework. Two dimensional approximated microstructures are generated assuming elliptic grains, based on the grain size, shape factor and area fraction of the primary Al phase of the said alloy at different processing condition. Plastic instability is shown using stress and strain distribution between the Al rich primary and Si rich eutectic phases under different boundary conditions. Boundary conditions are applied on the approximated RVEs in such a manner, so that they represent the real life situation depending on their position on a cylindrical tensile test sample. FE analysis is carried out using commercial finite element code ABAQUS without specifying any damage or failure criteria. Micro-level in-homogeneity leads to incompatible deformation between the constituent phases of the rheocast alloy and steers plastic strain localisation. Plastic stain localised regions within the RVEs are predicted as the favourable sites for void nucleation. Subsequent growth of nucleated voids leads to final failure of the materials under investigation.

Structural variations in aromatic 2 pi-electron three-membered rings of the main group elements

Structural variations of different Z pi-aromatic three-membered ring systems of main group elements, especially group 14 and 13 elements as compared to the classical description of cyclopropenyl cation has been reviewed in this article. The structures of heavier analogues as well as group 13 analogues of cyclopropenyl cation showed an emergence of dramatic structural patterns which do not conform, to the general norms of carbon chemistry. Isolobal analogies between the main group fragments have been efficiently used to explain the peculiarities observed in these three-membered ring systems.

Enhancing the mechanical and biological performance of a metallic biomaterial for orthopedic applications through changes in the surface oxide layer by nanocrystalline surface modification

Nanostructured metals are a promising class of biomaterials for application in orthopedics to improve the mechanical performance and biological response for increasing the life of biomedical implants. Surface mechanical attrition treatment (SMAT) is an efficient way of engineering nanocrystalline surfaces on metal substrates. In this work, 316L stainless steel (SS), a widely used orthopedic biomaterial, was subjected to SMAT to generate a nanocrystalline surface. Surface nanocrystallization modified the nature of the oxide layer present on the surface. It increased the corrosion-fatigue strength in saline by 50%. This increase in strength is attributed to a thicker oxide layer, residual compressive stresses, high strength of the surface layer, and lower propensity for intergranular corrosion in the nanocrystalline layer. Nanocrystallization also enhanced osteoblast attachment and proliferation. Intriguingly, wettability and surface roughness, the key parameters widely acknowledged for controlling the cellular response remained unchanged after nanocrystallization. The observed cellular behavior is explained in terms of the changes in electronic properties of the semiconducting passive oxide film present on the surface of 316L SS. Nanocrystallization increased the charge carrier density of the n-type oxide film likely preventing denaturation of the adsorbed cell-adhesive proteins such as fibronectin. In addition, a net positive charge developed on the otherwise neutral oxide layer, which is known to facilitate cellular adhesion. The role of changes in the electronic properties of the oxide films on metal substrates is thus highlighted in this work. This study demonstrates the advantages of nanocrystalline surface modification by SMAT for processing metallic biomaterials used in orthopedic implants.

Temperature Dependence of Mechanical Properties in Molecular Crystals

Quantitative evaluation of the mechanical behavior of molecular materials by a nanoindentation technique has gained prominence recently. However, all the reported data have been on room-temperature properties despite many interesting phenomena observed in them with variations in temperature. In this paper, we report the results of nanoindentation experiments conducted as a function of temperature, T, between 283 and 343 K, on the major faces of three organic crystals: saccharin, sulfathiazole (form 2), and L-alanine, which are distinct in terms of the number and strength of intermolecular interactions in them. Results show that elastic modulus, E, and hardness, H, decrease markedly with increasing T. While E decreases linearly with T, the variations in H with T are not so, and were observed to drop by similar to 50% over the range of T investigated. The slope of the linear fits to E vs T for the organic crystals was found to be around 1, which is considerably higher than the values of 0.3-0.5 reported in the literature for metallic, ionic, and covalently bonded crystalline materials. Possible implications of the observed remarkable changes in H for pharmaceutical manufacturing are highlighted.

Multi-transform based spectral element to include first order shear deformation in plates

For obtaining dynamic response of structure to high frequency shock excitation spectral elements have several advantages over conventional methods. At higher frequencies transverse shear and rotary inertia have a predominant role. These are represented by the First order Shear Deformation Theory (FSDT). But not much work is reported on spectral elements with FSDT. This work presents a new spectral element based on the FSDT/Mindlin Plate Theory which is essential for wave propagation analysis of sandwich plates. Multi-transformation method is used to solve the coupled partial differential equations, i.e., Laplace transforms for temporal approximation and wavelet transforms for spatial approximation. The formulation takes into account the axial-flexure and shear coupling. The ability of the element to represent different modes of wave motion is demonstrated. Impact on the derived wave motion characteristics in the absence of the developed spectral element is discussed. The transient response using the formulated element is validated by the results obtained using Finite Element Method (FEM) which needs significant computational effort. Experimental results are provided which confirms the need to having the developed spectral element for the high frequency response of structures. (C) 2015 Elsevier Ltd. All rights reserved.

Mechanical properties of CNT-Bisphenol E cyanate ester-based CFRP nanocomposite developed through VARTM process

This paper reports on the effect of multiwall carbon nanotubes (CNTs) without and with chemical functionalization on the mechanical properties of Bisphenol E cyanate ester resin (BECy) based carbon fibre reinforced plastic (CFRP) laminated composites. BECy with its low viscosity, low moisture uptake and superior mechanical properties is selected for its application in CFRP laminates through the cost-effective Vacuum Assisted Resin Transfer Moulding (VARTM) process. However, unlike CNT-epoxy-CFRP composites, processing and performance issues such as dispersion of CNTs, chemical bonding with resin, functionalization effects, effects on mechanical properties, etc. for BECy-CNT-CFRP composite system are not well reported. The objective of this study is to improve the mechanical properties of BECy resin with small additions of CNTs and functionalized CNTs in CFRP laminates. CNTs and fCNTs are infused into BECy using ultrasonication and standard mixing methods. Improvements in Young's modulus and strength in tension, compression, shear, flexure and interlaminar shear strength are analysed. It is observed that addition of 0.5wt% CNTs effected for maximum mechanical properties of the resin and 1wt% CNTs for the mechanical properties of CNT-CFRP nanocomposite. Further, improvements obtained with fCNTs are marginal. Dispersion behaviour and effect of CNTs/fCNTs in load transfer corroborated with SEM pictures are presented. The enhanced mechanical properties realized in VARTM processing of BECy-CFRP laminate indicate clear advantage of CNT based modification of the process.

Simultaneous enhancement in mechanical strength, electrical conductivity, and electromagnetic shielding properties in PVDF-ABS blends containing PMMA wrapped multiwall carbon nanotubes

A unique approach was adopted to drive the multiwall carbon nanotubes (MWNTs) to the interface of immiscible PVDF-ABS blends by wrapping the nanotubes with a mutually miscible homopolymer (PMMA). A tailor made interface with an improved stress transfer was achieved in the blends with PMMA wrapped MWNTs. This manifested in an impressive 108% increment in the tensile strength and 48% increment in the Young's modulus with 3 wt% PMMA wrapped MWNTs in striking contrast to the neat blends. As the PMMA wrapped MWNTs localized at the interface of PVDF-ABS blends, the electrical conductivity could be tuned with respect to only MWNTs, which were selectively localized in the PVDF phase, driven by thermodynamics. The electromagnetic shielding properties were assessed using a vector network analyser in a broad range of frequency, X-band (8-12 GHz) and Ku-band (12-18 GHz). Interestingly, enhanced EM shielding was achieved by this unique approach. The blends with only MWNTs shielded the EM waves mostly by reflection however, the blends with PMMA wrapped MWNTs (3 wt%) shielded mostly by absorption (62%). This study opens new avenues in designing materials, which show simultaneous improvement in mechanical, electrical conductivity and EM shielding properties.

Tailored mechanical behavior of magnetic particles loaded carbon nanotube foam in presence of magnetic field

The compressive behavior of carbon nanotube (CNT) foam with an entangled microstructure has become an important research area due to its excellent energy absorption capability. This report presents a tailored mechanical behavior of CNT foam under an applied magnetic field when all CNTs in the foam are coated with magnetic nanoparticles. The presence of nanoparticles not only enhanced the stiffness of the foam to four times but also revealed a nonlinear variation in both the stress and energy absorption capability with the gradual increase of the magnetic field. Magnetization of both CNT and attached nanoparticles along the magnetic field direction are shown to play a crucial role in determining the dominant deformation mechanism.

Hydrogen-induced mechanical properties of amorphous silicon thin films

Mechanical properties of thin films such as residual stress and hardness are of paramount importance from the device fabrication point of view. Intrinsic stress in sputtered films can be tensile or compressive as decided by the number density and the energy of the plasma species striking the growing film. In the presence of hydrogen we analyzed the applicability of idealized stress reversal curve for amorphous silicon thin films deposited by DC, pulsed DC (PDC) and RF sputtering. We are successfully able to correlate the microstructure with the stress reversal and hardness. We observed a stress reversal from compressive to tensile with hydrogen incorporation. It was found that unlike in idealized stress reversal curve case, though the energy of plasma species is less in DC plasma, DC deposited films exhibit more compressive stress, followed by PDC and RF deposited films. A tendency towards tensile stress from compressive stress was observed at similar to 13, 18 and 23 at%H for DC, PDC and RF deposited films respectively, which is in exact agreement with the vacancy to void transition in the films. Regardless of the sputtering power mode, the hardness of a-Si:H films is found to be maximum at C-H similar to 10 at%H. Enhancement in hardness with C-H (up to C-H similar to 10 at%H) is attributed to increase of Si-H bonds. Beyond C-H similar to 10 at%H, hardness starts falling. (C) 2015 Elsevier Ltd. All rights reserved.

Anisotropy in the mechanical properties of organic crystals: temperature dependence

The nanoindentation technique has recently been utilized for quantitative evaluation of the mechanical properties of molecular materials successfully, including their temperature (T) dependence. In this paper, we examine how the mechanical anisotropy varies with T in saccharin and L-alanine single crystals. Our results show that elastic modulus (E) decreases linearly in all the cases examined, with the T-dependence of E being anisotropic. Correspondence between directional dependence of the slopes of the E vs. T plots and the linear thermal expansion coefficients was found. The T-dependence of hardness (H), on the other hand, was found to be nonlinear and significant when (100) of saccharin and (001) of L-alanine are indented. While the anisotropies in E and H of saccharin and E of L-alanine enhance with T, the anisotropy in H of L-alanine was found to reduce with T. Possible mechanistic origins of these variations are discussed.